Giant ichneumon wasps and something strange

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MarkSturtevant
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Giant ichneumon wasps and something strange

Post by MarkSturtevant »

Giant ichneumon wasps (genus Megarhyssa) are large parasitic wasps that possess a remarkably long ovipositor -- the longest of any insect. Females use this tool to drill deep into wood in order to parasitize the wood-boring larvae of another wasp known as the horntail.

During the previous summer I came across about a dozen of these wasps that were busily drilling into the wood of a dead tree. I have never seen a large aggregation like this, so it was very exciting. Both of our local species were represented, including Megarhyssa macrurus, as shown below...
ImageGiant Ichneumon wasp by Mark Sturtevant, on Flickr

... and the handsomely attired M. atrata.
ImageGiant Ichneumon wasp by Mark Sturtevant, on Flickr
You should note that there is a small wasp that is also in the above picture. That turns out to be the "strange thing", as you will see later.

The next picture shows an M. macrurus again. The large area of membrane on the abdomen appears during a certain phase in the drilling process. It will retract later as she continues to drill into the wood, eventually driving her great ovipositor to the 'hilt' of her abdomen.
ImageGiant Ichneumon wasp by Mark Sturtevant, on Flickr
As I understand it, she can detect the host larva by either smell or by sound (there seems uncertainty about that), but she will be able to insert an egg directly into the victim that is buried deep into the wood. Incredible.

Here is a male giant ichneumon. There were several of these wandering up and down the tree as well.
ImageGiant Ichneumon wasp by Mark Sturtevant, on Flickr

The females would often group close together to parasitize the same horntail larva. On this occasion an M. atrata was minding her own business, but M. macrurus came down, stood over her, and if you look carefully you can see she is inserting her ovipositor into the same hole that was being used by M. atrata! This "claim jumping" is pretty common, I suspect.
ImageGiant Ichneumon wasps by Mark Sturtevant, on Flickr

Meanwhile there was that little wasp that was mentioned above. After a time, I noticed that it was firmly "riding" a female M. atrata!
ImageA mystery visitor on the "Ichneumon Tree" by Mark Sturtevant, on Flickr

Here are some more pictures of this strange scene.
ImageThe mystery visitor is persistent. by Mark Sturtevant, on Flickr
ImageThe mystery visitor watches by Mark Sturtevant, on Flickr
The giant ichneumon was clearly annoyed by its little rider, and it was trying to swipe it away, but the smaller wasp stayed out of reach and was determined to stay put. Eventually, the big female began to drill while the smaller wasp seemed to watch intently.

What was going on? I asked this question to the folks at BugGuide. Their consensus is that the little wasp was a male ichneumon of another genus (Rhyssella). Some species in this group also parasitize horntails, and so they too may be attracted to this tree. What was going on here might be a case of inter-species confusion. Male wasps are known to be very amorous, and they will attempt to mate with females of other species. If so, this would certainly be a very ambitious but deeply confused male!
Mark Sturtevant
Dept. of Still Waters

rjlittlefield
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Post by rjlittlefield »

Wonderful images and story!

Having just read Eisner's book, I am primed to wonder if the wasps identify potential mates by smell, and the little male was attracted to the giant female because she "just smelled right" ?

--Rik

Lou Jost
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Post by Lou Jost »

Fantastic photos and story!

Olympusman
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Wasps

Post by Olympusman »

Incredible series!

Mike
Michael Reese Much FRMS EMS Bethlehem, Pennsylvania, USA

Troels
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Post by Troels »

Fantastic creatures and pictures. Very interesting.
Troels Holm, biologist (retired), environmentalist, amateur photographer.
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MarkSturtevant
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Post by MarkSturtevant »

rjlittlefield wrote:Wonderful images and story!

Having just read Eisner's book, I am primed to wonder if the wasps identify potential mates by smell, and the little male was attracted to the giant female because she "just smelled right" ?
--Rik
I can't be certain, but the other explanation seems less likely. For a time I was all excited by the possibility that I was witnessing a bit of "hyperparasitism", which is where a parasite parasitizes another parasite. But that began to fall apart with my conversation in BugGuide. The wasp strongly resembles species that parasitize horntails, and then the point that this is a MALE wasp, not a female, and that male wasps can be rather oblivious about what or whom they mount. This also extends to male bees, by the way, with weird examples like male bees copulating with orchid flowers.
Gotta admit that made the previously described scenario more likely, though a bit less exciting to me.
Mark Sturtevant
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JH
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Post by JH »

Interesting, thanks for posting.
Best regards
Jörgen Hellberg
Jörgen Hellberg, my webbsite www.hellberg.photo

ChrisR
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Post by ChrisR »

Brilliant, Mark, - well done!
Chris R

hayath
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Post by hayath »

Brilliant images, these!

Amazing patience to document all of it

Bob-O-Rama
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Post by Bob-O-Rama »

That's just an amazing set. I've never see that membranous structure before, very interesting. As for the freeloader, maybe this is Wasp Uber?

Picosvistas
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Post by Picosvistas »

Superb set of images!

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